This invention is concerned with plugs for use in securing a screw in a previously formed hole in a work piece.
Plastics plugs, which are inserted into a hole in a work piece, to receive a conventional screw to secure the screw to the work piece are well known.
It is usual to use different plastics plugs for different work pieces; in particular, for securing screws into a blind hole in a hard work piece, such as brick or concrete; into a blind hole in a soft friable work piece, such as blown concrete; or, into a hole extending right through a sheetform work piece.
Attempts have been made to provide a plug which is suitable for use in all these three situations, but the requirements for a fastener to work satisfactorily in all three conflict with one another.
For a plug which is to secure a screw in a hole in a rigid work piece, the plug is conventionally of uniform outside diameter so that it may readily be inserted in the hole. A central bore, to receive the screw is usually provided with ridges so that on the screw being driven into the plug the screw engages the plug firmly, and a small amount of expansion of the plug takes place to grip the hole firmly. Use of such a plug in a friable material will usually not provide a firm fixture as the expansion of the plug is inadequate to hold the plug securely in the material.
For a plug which is to secure a screw in a hole through a sheetform material, the plug usually comprises a tail portion connected to a head portion of the plug by a series of legs, and the screw is intended to pass freely through the bore of the plug until it reaches the tail portion, and then to draw the tail portion towards the head portion to force the legs to bend outwardly to secure the plug to the work piece. It will be realized that the provision of ridges in the bore of the plug (to enable the plug to be used, as above, in rigid material) will prevent easy passage of the screw from the head portion to the tail portion.
Moreover, where the plug is constructed from plastics material it is difficult and generally not cost effective to form a thread in the bore of the tail portion which would assist passage of the screw. Thus, in plastics plugs, the screw normally cuts its own thread and of course this offers considerable resistance to the turning of the screw. The screw can therefore impose a severe torque on the tail portion while it is being driven. This torque can be sufficient to twist the legs and indeed this is the intended result in some plugs presently available, so that the legs twist around the screw and, as the screw draws the tail portion towards the head portion the legs are squeezed into a ball formation. However, this places very localised pressures around the edge of the hole in the sheetform material so that where the latter is plaster board or the like the edge may crumble and prevent a secure fixing being made.
In WO/9008265, there is disclosed a plug where this tendency of the legs to twist is countered. This plug comprises a head portion having an enlarged flange adapted to engage the work piece and a generally cylindrical portion, tapering outwards slightly from the flange, a cylindrical tail portion of substantially the same diameter as the smallest part of the head portion and four legs interconnecting the tail portion and the head portion.
These legs are in two pairs, one pair of opposed legs of approximately a semicircular cross section, and another pair of opposed legs of somewhat smaller cross section. The larger legs are arranged such that twisting of them is substantially prevented, and only folding at defined weakened positions is essentially possible.
This plug will operate successfully in a hole in rigid material, and will also operate in a hole in sheetform material of appropriate thickness, where the legs will fold outwards near their middles so that the halves of the legs connected to the head portion are levered flat against the back of the sheetform work piece as the tail portion is drawn towards the head portion on driving of the screw into the tail portion. A secure fixing, even in friable material such as plasterboard, is therefore possible because the pressures on the board are spread by the legs being flat against the board, and are not concentrated on the edge of the hole.
Even so, the screw having to cut its own thread in the tail portion (despite the provision of ridges in the tail portion to reduce the requisite torque required) nevertheless makes the plug difficult to set and offers considerable resistance to driving of the screw. Indeed, although the aforementioned ridges do reduce the torque required, this reduction is at the expense of the security of the fixing; it is quite possible to overdrive the screw without appreciating that in so doing the threads painstakingly cut in the tail portion have now been stripped.
However, it is in soft friable material where the plug will not generally operate satisfactorily: the plug will not expand adequately into the friable material simply on insertion of a screw into the plug, and if the screw is driven further, the rigidity of the larger pair of the legs is such that only a small amount of lateral expansion, in a manner comparable to that taking place when used with sheet material, will take place, and a secure fastening cannot be ensured. In this context, the aforementioned twisting of the legs into a ball is desirable as it expands into the friable material.
A solution to this problem is to make the legs essentially the same so that twisting is possible, but provide threads in the tail portion so that less torque is required to drive the screw into the tail portion (but without reducing the strength of the connection between the screw and tail portion) so that the undesirable ball formation does not occur, or is not so likely to occur, when the plug is used in the sheetform work piece situation.
However, while there is little problem in forming threads in plugs of this type, it is a different matter to do so in a cost effective manner.
U.S. Pat. No. 4,547,108 discloses a trim cover clip which has a thread grip moulded on an internal flexible tongue formed in a bore of the clip. Here a core mould passes through a window of the clip. However, the thread so-formed is only single sided and in order to grip a screw must be biassed towards the screw and accordingly offers significant resistance to entry of the screw.
It is an object of the present invention to provide a plug of plastics material having moulded thread formations, and a method of moulding same which does not suffer the aforementioned disadvantages, or at least mitigates their effects.